Tachometer

ABSTRACT

There is provided a novel, solid state, tachometer with a novel, light-emitting diode (LED) display.

United States Patent Olson Dec. 17, 1974 I TACHOMETER 3,603,879 7/1971 Pelta et al 324/169 [76] Inventor: John M. Olson 1513 7th St" Port 3,758,858 9/1973 McCue 324/78 1 Hueneme, Calif. 9304] OTHER PUBLICATIONS [22] Filed. Apr 2 1973 Information Displays: Innovation Is The Rule by R. K.

Jurgen IEEE Spectrum Nov. 1972, Vol. 9, No. l l, p. 18.

Primary Examiner.lohn W. Caldwell 52 U. Cl 4 Cl l I u 340/52 g ggi Assistant ExammerMarshall M. Curtis [58] Field of Search 324/169 78 J 340/324 R premch 340/324 A 6 52 R 52 Wasserman, Rosen & Fernandez [56] References Cited [57] ABSTRACT UNITED STATES PATENTS There is provided a novel, solid state, tachometer with 3,369,178 2/1968 Wilson 324/78 J a novel hght'em'ttmg mode (LED) dlsplay 3,469,252 9/1969 Bet 340/324 A 10 Claims, 3 Drawing Figures I567 58 255E? souaea OF 62 6O 54 HHTH HTTHH T 46 1 v 48 DRIVERS 52 THWF THHTH A A 4 51-11FT Rae E HIFT REG 01. R T CL R IGNITION SYSTEM 40 souRcE OF TIMER L- 32m POTENTIAL CIRCUIT 11 VIBRATOR SENSORS cuzcun' Pmmmmm H 1 TRE5HOLD OF \NV 2.6

56R GATE TR\GGF R D L O H 5 E R H T INPUT INPUT VOLTAE AT BASE AND EMHTER 22 OUTPUT TACHOMETER BACKGROUND OF THE INVENTION This invention relates to tachometers for indicating RPM of an engine, and more particularly, to improvements therein.

OBJECTS AND SUMMARY OF THE INVENTION An object of this invention is the provision of a novel, completely solid state tachometer.

Another object of this invention is the provision of a solid state tachometer with a unique display.

Still another object of this invention is the provision of a tachometer which operates linearly and has no lag or overshoot.

Yet another object of this invention is the provision of an improved tachometer input circuit arrangement which permits direct interface with all standard ignition systems (conventional, CD, magneto, etc.). generates pulses.

These and other objects of the invention may be achieved in an arrangement for a tachometer wherein the signals derived from the distributor are applied to an input circuit which, in response, generates. These pulses constitute the clock pulse input to a shift register counter, and also the first of a number of these pulses, causes a timer circuit to generate a timing pulse whose width constitutes a time base signal during which incoming clock pulse signals are counted.

The leading edge of the timing pulse is used to reset the shift register counter, which can thereafter begin to count clock pulses. Each stage of the shift register counter has its output connected to a silicon controlled rectifier, (SCR), so that when the shift register stage represents a l, the SCR is rendered conductive. Each SCR is in series with a light-emitting diode (SCR) which is turned on when the SCR becomes conductive. The traling edge of the timing pulse is used to momentarily remove operating potential from the SCRs. This will cause them to become nonconductive. Whe the operating potential is turned on again, only those of the SCRs which are connected to shift register stages with a count of 1 are turned on again. The LEDs provide an illuminated indication of the engine RPM.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block schematic diagram of an embodiment of the invention.

FIG. 2 is a waveform diagram, which is shown to assist in an understanding of the operation of the input circuit of FIG. 1.

FIG. 3 is a representation of the appearance of a tachometer display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there may be seen a block schematic diagram of a preferred arrangement for this invention. The input circuit of the tachometer circuit is connected to the ignition system of the engine whose RPM is desired to be displayed. The tachometer input circuit comprises a silicon controlled rectifier l2 (SCR) whose gate electrode receives output from the ignition system. The cathode of the SCR is connected to ground, and the anode is connected through a series connected resistor, 14, to a source of operating potential 18. A capacitor 20 is connected across the SCR.

A transistor 22 has its baseconnected to the SCR anode and to one side of the capacitor 20. The collector of the transistor is connected to the operating potential 18. The emitter of the transistor is connected through a resistor 24 to ground. This transistor serves as an emitter follower. The emitter of transistor 22 is connected to an inverter 26, which is biased from a source of potential 27, so that it acts as a level detector.

The tachometer input circuit which is shown, utilizes the high frequency components of the input signal. This technique allows direct interface with all standard ignition systems, since conventional tachometers filter out the high frequency component in order to trigger on the DC level shift. As a result, with the prior art tachometers, a special tachometer must be purchased if a person changes to an engine with a magneto ignition system, for example, whereas, with the present arrangement, the one tachomeer can be used with all ignition systems.

Capacitor 20 charges up from the source of operating potential 18 through resistor 14 to a voltage level. A threshold voltage must be exceeded by the signal applied to its gate electrode in order to enable the SCR 12 to become conductive. When this occurs, the capacitor charge is dissipated through the SCR. This effectively turns off the SCR so that it can be triggered by the next input signal which exceeds the gate threshold voltage. The emitter follower transistor 22 acts as an impedance converter, converting the high impedance of the capacitor-SCR circuit into a low impedance which can drive the biased inverter 26. Transistor 22 output effectively follows its input.

Referring now to FIG. 2, there are shown waveforms which assist in an understanding of the operation of the input circuit. The waveform A is obtained upon opening of a pair of ignition contacts. This waveform resembles a damped sine wave. Whenever any part of the damped sine wave exceeds the indicated threshold level, the SCR is triggered.

The SCR gate trigger voltage is a waveform with the appearance of waveform B. This causes discharge and charge of the capacitor 20 which provides a sawtooth waveform, such as waveform C, at the base and emitter of transistor 22. The threshold level of the biased inverter established by bias source 27 is represented by the dotted line on the drawing. It will be seen that during the interval of the occurrence of the damped sine wave train A, the voltage applied to the input of the biased inverter does not exceed this threshold level. This does not occur until the time required for capacitor 20 to charge from its discharge level up to the threshold of the inverter 26. This time can be established by the values of the components selected to be used. By way of example, this time was established as 600 ts to allow the damped sine wave train to essentially die out.

Waveform D represents the output of the biased inverter 26. This is a pulse which has a positive going leading edge and a negative going trailing edge. The trailing edge occurs when the input to the inverter exceeds the level of the bias established by source 27.

Effectively, the input circuit described constitutes a retriggerable one-shot circuit which generates pulses in response to inputs of the type typically generated by an ignition circuit.

The positive going output of inverter 26 is inverted by an inverter 28. The output of inverter 28 is applied to an inverter 30, and also to a timer circuit 32. The timer circuit provides a time base signal during which incoming signals from the input circuit may be counted. The period of the timer is determined by the display requirements, that is, its period or pulse width is equal to the period of the RPM increment being displayed. For example, if a tachometer for an 8 cylinder engine is to read in 500 RPM increments, then the timer pulse width should be 30 milliseconds. This may be seen from the fact that an 8 cylinder engine produces 4 pulses per revolution. At 500 RPM, 2000 pulses per minute would be provided. 2,000 pulses per minute equals 33.3 pulses per second equals frequency, f.The timer period equals l/f, equals 30 milliseconds.

The timer is activated by a negative going transition at its input. This is the leading edge of the input pulse. Once triggered, additional input pulses have no effect on the timer until after the termination of the timer output. The timer circuit will be recognized by those skilled in the art, as either a one-shot multivibrator circuit, or an operational amplifier with suitable feedback and biased to insure that its output is a pulse having the desired pulse width.

The output of the timer, which is a positive going pulse is applied to an inverter 34, and through a circuit consisting of series connected capacitor 36, and parallel connected resistor 38, to an inverter 40. The leading edge of the pulse output of the timer is inverted by the inverter 40 to provide a negative going signal, which is required to reset two series connected shift registers, respectively 42, 44.

These shift registers, respectively 42, 44, operate as counters, by shifting the level 1 signals presented at the stage at one end of the shift register 42, designated as the input stage, to succeeding shift register stages in response to the pulses which are received from inverter 30, which are now designated as clock pulses. The inputs to shift registers 42 and 44 are designated as A and B inputs. Both inputs must be high, in order to shift a 1 into the register at the clock pulse positive going transition. The input A is hard wired into a high stage by being connected to the source of operating potential 18.

The B inputs to the shift registers are connected to an alarm multivibrator 46. The alarm multivibrator normally provides a high signal output to the B inputs of the shift registers. The input to the alarm multivibrator may be connected to one or more sensors 47 on the engine, such asoil pressure, or water temperature, or battery fluid, whereby, when the sensor determines that temperature is too high, or oil pressure is low, it grounds the input to the alarm multivibrator causing it to provide a sequence of alternate 1's and Os at its output, at a predetermined low frequency, such as 1 Hz. Since this output is connected to the B inputs to the registers, and since the display is updated every 30 to 60 ms (depending on input RPM), the output display, which is connected to the registers would be caused to flash on and off, at the frequency of 1 Hz thus indicating something is wrong, which would be a signal to turn off the engine.

When the engine is performing normally however, the A and B inputs to the shift register 42 are high, whereby its serial input would be set in a l representative state, and 1s would be shifted in response to clock pulses along shift register 42. Because of the time base established by the timer circuitpulse, one clock pulse is counted when the engine rotates at 500 RPM, two at 1,000 RPM, etc. It is to be noted that the A input of shift register 44 is connected to the last stage of the shift register 42. Thus, when shift register 42 fills, if there are additional clock pulses, shift register 44 begins to shift ls along its length.

The outputs of the shift register stages enable SCRs to become conductive. One SCR may be provided, for each stage of a shift register. By way of example, SCR 46 is shown having its gate electrode connected through a current limiting resistor 48, to the output of the first stage of the shift register 42. If desired, two diodes respectively 50, 52, connected in series, may be connected from the cathode of all of the SCRs to ground providing a reverse gate bias. The anode of each SCR is connected in series with an LED 54. The anode of each LED is connected through a current limiting resistor 56, to the collector of a transistor 58.

LED 54 is typical of the connections of all of the LEDs. They all have current limiting resistors such as 56, connected to a common bus, which is connected to the collector of transistor 58. The emitter of transistor 58 is connected to the source of operating potential. The output of inverter 34 is connected through a differentiating circuit, comprising series connected capacitor 60 and shunt connected resistor 62, to the base of transistor 58.

When a stage of a shift register represents a I, it enables the SCR connected to its output to become con-' ductive. As a result, operating potential is supplied through transistor 58, through an LED to the SCR, whereby the LED becomes illuminated. As previously indicated, the output of the timer 32 is a positive going pulse, which is inverted by an inverter 34. The differentiating circuit comprised of capacitor 60 and resistor 62, causes a positive going signal to be applied to the base of transistor 58, in response to the trailing edge of the timer circuit output. This momentarily renders transistor 58 nonconductive, whereby operating potential is removed from all of the LEDs and SCRs which are connected thereto. This causes a turn off of all of the SCRs and their associated LEDs. When transistor 58 is turned on again, which occurs after the passage of the trailing edge of the timer circuit pulse, only those SCRs and associated LEDs will be turned on where shift register stages have a l representation.

In summary of the operation of the circuit arrange. ment shown in FIG. 1, signal outputs from the ignition system, which may constitute damped sine wave trains, are applied to an input circuit, which generates clock pulses in response. A first of these clock pulses causes a timer circuit to generate a pulse having a width sufficient to enable a number of the clock pulses as determined by engine RPM to be applied to the shift register to be counted. The trailing edge of the timer output pulse resets the display by momentarily rendering transistor 58 nonconductive, so that as soon as the transistor is enabled again, the display, will represent the contents of the shift register counter. Clock pulses are counted over the interval of the timer pulse width, and the display panel will display the total count which occurs before the occurrence of the next timer pulse.

Even though the shift register counter is reset at the start of the timer circuit pulse, the SCRs will maintain the LEDs illuminated, since they are not reset until the trailing edge of the timer pulse. Thus, the SCRs operate as a memory storing the preceding count until a new count is made. The LEDs present a display of the RPMs of the engine which is continuously being updated.

If desired, outputs may be taken from the SCRs for other purposes besides illuminating the LEDs. For example, if it is desired to provide a warning as the engine RPM approaches a dangerous speed, or if it is desired to limit engine RPM, an output may be taken from the sCR which becomes conductive at the RPM at which such effect is desired, and this output may be used to light a warning light or actuate a solenoid.

FIG. 2 is an illustration of the appearance of the display panel for the embodiment of the invention. A panel 170 has openings therein through which each one of the LEDs 50 appears. Numbers are printed adjacent to LEDs which indicate RPM in thousands. The LEDs produce sufficient light so that they can be readily seen in daylight. By looking at the dial, and more specifically by seeing the number of illuminated LEDs, one can readily determine his RPM, so that the driver knows whether his engine is turning over fast enough to enalbe shifting into a higher gear. Should a problem occur, such as was indicated with the water temperature or oil pressure, the LEDs will flash on and off, thus indicating that something is wrong. This immediately calls the attention of the driver to this fact, and he can shut off the engine promptly, and thereby avoid seriously damaging the engine. I

Effectively, the display is a very easy reading arrangement since all that need be noted is the length of the illumination to know what the RPM of the engine is. If it is desired to provide a folded scale display, i.e., a display that compresses the low RPMs but expands for the high RPMs, this can be done for example, by inserting a counter stage in front of the shift register 42 which will count for 3,000 RPM before enabling the shift register to count. Only one LED maybe used for indicating the 3,000 RPM count, the higher RPM counts are then shown in the manner indicated.

There has accordingly been described and shown herein a novel and useful solid state tachometer. The tachometer operates linearly and is free from overshoot or lag.

. What is claimed is:

1. A tachometer for indicating RPM of an engine from engine revolution representative signals generated by said engine comprising:

a display comprising light-emitting diode means,

timer means for establishing a timer pulse signal having a predetermined duration,

means responsive to said engine revolution representative signals occurring during said timer pulse signal duration, for illuminating said light-emitting diode means to represent the RPMs of said engine, and

means for momentarily quenching said light-emitting diode means at the ending of one of said timer pulse signals.

2. A tachometer as recited in claim 1 whererin said means for illuminating of said light-emitting diode means includes:

counter means,

means responsive to the leading edge of a timer pulse signal to reset said counter,

means responsive to said engine revolution representative signals to generate clock pulses, means for applying said clock pulses to said counter to be counted during said timer pulse signal duration, and A means responsive to the count of said counter means for illuminating said light-emitting diode means to represent the number of RPMs of said engine. 3. A tachometer as recited in claim 2 wherein said light-emitting diode means comprises a plurality of LEDs.

wherein said means responsive to the count of said counter means for causing illumination of said light-emitting diodes for representing the number of RPMs of said engine comprises: i

a plurality of SCRs,

means for connecting the gate electrodes of said SCRs to said counter means for enabling the number of said plurality of SCRs to become conductive which represent the count in said counter means,

means connecting each of said plurality of LEDs in series with a different one of said plurality of SCRs, and

means for applying operating potential for each LED and SCR across all of said SCRs and LEDs.

4. A tachometer as recited in claim 1 wherein there is included means for sensing an undesirable condition in said engine and producing an output indicative thereof, and

means responsive to said output for causing said light-emitting diode means to flash on and off. 5. A tachometer for indicating the RPM of an engine from engine revolution representative signals comprismeans responsive to said engine revolution representative signals for generating clock pulses, timer means responsive to one of said clock pulses for generating a timer pulse having a duration extending over the interval of occurrence of a plurality of said clock pulses,

counter means to which said clock pulses are applied to be counted,

means responsive to the start of a timer pulse for resetting said counter means,

a plurality of SCRs,

means connecting said plurality of SCRs to said counter means for rendering conductive the number of said plurality of SCRs which represent the count in said counter means,

a display means,

means to activate said display means responsive to said conductive SCRs to display an indication of engine RPM, and

means responsive to the ending of a timer pulse to momentarily render nonconducting all of the conductive SCRs.

6. A tachometer as recited in claim 5 wherein said display means comprises a LED for each SCR operated responsive thereto.

7. A tachometer as recited in claim 6 wherein each LED is connected in series with a different one of said SCRs,

wherein said means to actuate said display means responsive to said conductive SCRs includes:

a source of operating potential, and

means applying said engine revolution representative signals to said SCR gate electrode,

a capacitor connected between said SCR anode and cathode,

a transistor having base, emitter and collector electrodes,

means connecting said base electrode to said SCR anode,

a first resistor connected between said transistor base and said transistor collector,

a second resistor connected to said transistor emitter,

means to apply operating potential across said transistor collector and said SCR cathode and the other end of said second resistor, and

biased means connected to said emitter electrode to produce a clock signal when the level of the signal at said transistor emitter exceeds a predetermined threshold.

9. A tachometer as recited in claim 5 wherein there is included sensor means for sensing an undesirable condition in said engine and producing an output indicative thereof, and

oscillator means responsive to said output for causing said display means to signal said undesirable condition.

10. A tachometer as recited in claim 9 wherein said counter means comprises shift register means having a plurality of stages,

said oscillator means comprises an oscillator which is rendered operative responsive to said sensor output, and

means for applying the output of said oscillator means to the input to said shift register means to cause said display means to flash on and off responsive thereto when said oscillator means is operative. 

1. A tachometer for indicating RPM of an engine from engine revolution representative signals generated by said engine comprising: a display comprising light-emitting diode means, timer means for establishing a timer pulse signal having a predetermined duration, means responsive to said engine revolution representative signals occurring during said timer pulse signal duration, for illuminating said light-emitting diode means to represent the RPMs of said engine, and means for momentarily quenching said light-emitting diode means at the ending of one of said timer pulse signals.
 2. A tachometer as recited in claim 1 whererin said means for illuminating of said light-emitting diode means includes: counter means, means responsive to the leading edge of a timer pulse signal to reset said counter, means responsive to said engine revolution representative signals to generate clock pulses, means for applying said clock pulses to said counter to be counted during said timer pulse signal duration, and means responsive to the count of said counter means for illuminating said light-emitting diode means to represent the number of RPMs of said engine.
 3. A tachometer as recited in claim 2 wherein said light-emitting diode means comprises a plurality of LEDs. wherein said means responsive to the count of said counter means for causing illumination of said light-emitting diodes for representing the number of RPMs of said engine comprises: a plurality of SCRs, means for connecting the gate electrodes of said SCRs to said counter means for enabling the number of said plurality of SCRs to become conductive which represent the count in said counter means, means connecting each of said plurality of LEDs in series with a different one of said plurality of SCRs, and means for applying operating potential for each LED and SCR across all of said SCRs and LEDs.
 4. A tachometer as recited in claim 1 wherein there is included means for sensing an undesirable condition in said engine and producing an output indicative thereof, and means responsive to said output for causing said light-emitting diode means to flash on and off.
 5. A tachometer for indicating the RPM of an engine from engine revolution representative signals comprising: means responsive to said engine revolution representative signals for generating clock pulses, timer means responsive to one of said clock pulses for generating a timer pulse having a duration extending over the interval of occurrence of a plurality of said clock pulses, counter means to which said clock pulses are applied to be counted, means responsive to the start of a timer pulse for resetting said counter means, a plurality of SCRs, means connecting said plurality of SCRs to said counter means for rendering conductive the number of said plurality of SCRs which represent the count in said counter means, a display means, means to activate said display meAns responsive to said conductive SCRs to display an indication of engine RPM, and means responsive to the ending of a timer pulse to momentarily render nonconducting all of the conductive SCRs.
 6. A tachometer as recited in claim 5 wherein said display means comprises a LED for each SCR operated responsive thereto.
 7. A tachometer as recited in claim 6 wherein each LED is connected in series with a different one of said SCRs, wherein said means to actuate said display means responsive to said conductive SCRs includes: a source of operating potential, and transistor means for applying, when conductive, operating potential from said source across said series connected SCRs and LEDs, wherein said means responsive to the ending of a timer pulse to momentarily render nonconducting all of the conductive SCRs includes means responsive to the trailing edge of said timer pulse for momentarily rendering said transistor means nonconductive.
 8. A tachometer as recited in claim 5 wherein said means responsive to said engine revolution representative signals for generating clock signals comprises: an SCR having gate, anode and cathode electrodes, means applying said engine revolution representative signals to said SCR gate electrode, a capacitor connected between said SCR anode and cathode, a transistor having base, emitter and collector electrodes, means connecting said base electrode to said SCR anode, a first resistor connected between said transistor base and said transistor collector, a second resistor connected to said transistor emitter, means to apply operating potential across said transistor collector and said SCR cathode and the other end of said second resistor, and biased means connected to said emitter electrode to produce a clock signal when the level of the signal at said transistor emitter exceeds a predetermined threshold.
 9. A tachometer as recited in claim 5 wherein there is included sensor means for sensing an undesirable condition in said engine and producing an output indicative thereof, and oscillator means responsive to said output for causing said display means to signal said undesirable condition.
 10. A tachometer as recited in claim 9 wherein said counter means comprises shift register means having a plurality of stages, said oscillator means comprises an oscillator which is rendered operative responsive to said sensor output, and means for applying the output of said oscillator means to the input to said shift register means to cause said display means to flash on and off responsive thereto when said oscillator means is operative. 